1. Field of the Invention
The present invention relates to a real time counter for a video cassette recorder (hereinafter referred to as "VCR"). More particularly the present invention relates to a control apparatus of a real time counter for a VCR which is capable of controlling precisely the operation of the real time counter which counts the degree of videotape travel, especially in the miniaturized 8-mm VCR format.
2. Description of the Prior Art
In the 8-mm VCR format, there are several methods for counting the degree of videotape travel: a method of real time counting, a method of counting the number of pulses from a take-up reel reflection plate, a method of measuring a remaining amount of videotape, etc.
Among them, the method of real time counting accomplishes a counting operation by detecting whether or not recorded signals exist on a videotape. It starts to count the degree of videotape travel by counting the recorded portion of the videotape from the zero point, i.e., the point where a "reset" signal is applied.
Alternatively, as a means for controlling a real time counter in a VHS type VCR, the operation of the real time counter is controlled by detecting a control signal which is marked earlier when recording an image.
On the other hand, in the 8-mm VCR format, instead of detecting the control signal, a video (RF signal Frequency) signal whose voltage level is detected when a head scans a videotape is used for judging whether recorded signals exist on the videotape. If the recorded signals exist, the operation of the real time counter is achieved by counting the number of the pulse signals generated by a frequency generator in a capstan motor. If there are no recorded signals, the operation of the real time counter is suspended.
FIG. 1 is a block diagram of a conventional control apparatus of a real time counter for a VCR. FIG. 2 shows waveforms appearing over the alphabetized parts (A-E) of the diagram in FIG. 1.
As shown in FIG. 1, a conventional control apparatus is composed of a CLOG detection section 30 for detecting a CLOG signal which is used for judging whether recorded signals from an RF signal amplified at a pre-amplifier 2 after reproduced by a head exist on the videotape; an AND gate 11 for performing a logical product between the CLOG signal and an REC signal which is used for choosing the mode of record or reproduction; a capstan motor 6 for allowing the videotape to travel; and a counting section 40 employing a real time counter 9 for counting the number of the pulse signals generated from a frequency generator in a capstan motor 6 of which the operation is controlled according to the output of the AND gate 11, and also employing a switch 8 for controlling the operation of the real time counter 9 according to the output of the AND gate 11.
In operation of the CLOG detection section 30, an RF signal which is reproduced by the head installed on the drum 1 is amplified at the pre-amplifier 2, and the envelope of the amplified signal is detected by the envelope detector 3 as a waveform (A) in FIG. 2. The envelope signal (the output of the envelope detector 3) is then compared at a comparator 4 with a reference voltage Vcref for judging whether a signal exists or not, and the compared output signal is shown as a waveform (B) in FIG. 2. As shown, if the level of the envelope signal, see the waveform (A) in FIG. 2, is higher than that of the reference voltage Vcref, the output of the comparator 4 goes `low`, while if the level of the envelope signal is lower than that of the reference voltage Vcref, the output of the comparator 4 goes `high`. At this time, the voltage value of the reference voltage Vcref can be arbitrarily set by a formula, "R.sub.2 /R.sub.1 +R.sub.2)," namely the voltage divider.
The output of the comparator 4 then passes through a low-pass filter 5 to make a waveform (C) depicted in FIG. 2. As shown, it is to be understood that the low-pass filter 5 suppresses a narrow-pulse-width signal, or a high-frequency component. The output of the low-pass filter 5 (the waveform (C) in FIG. 2) is provided as a CLOG signal to the following AND gate 11.
The AND gate 11 performs a logical product between a recording mode instruction, or an REC instruction and the output of the low-pass filter 5, or the CLOG signal. The REC instruction is set `low` in a recording mode, and is set `high` in the rest mode except during a record mode. The output of the AND gate 11 serves as a control signal for controlling the operation of the switch 8. If the control signal is low, the switch 8 is turned on, while if it is high, the switch 8 is turned off.
The real time counter 9 in the counting section 40 counts the extent of videotape travel by counting the number of CLOG signal pulses, see waveform (D) in FIG. 2, which is generated from the frequency generator in the capstan motor 6, and its form is shaped at a waveform shaper 7. The frequency of the CLOG signal is proportional to the revolutions of the capstan motor 6, and the CLOG signal is generated without interruption as long as the capstan motor 6 rotates.
The operation of the conventional control apparatus of the real time counter constructed above will be discussed both while in record mode and at rest.
In record mode, because the REC instruction has been set `low`, the output of the AND gate 11 goes `low` regardless of the state of the CLOG signal. Therefore the switch 8 always stays turned on, and the real time counter 9 continues to perform a counting operation.
In the rest mode except during record mode, because the REC instruction has been set `high`, the output of the AND gate 11 depends upon the state of the CLOG signal. As is poorly the case, even though there exists, see the waveform (A) in FIG. 2, the envelope signal, because some portions of the envelope signal, as periods P.sub.1, and P.sub.2, are under the reference voltage Vcref, the CLOG signal is provided to the AND gate 11 in a high level, and thereby the output of the AND gate 11 goes `high`. Thus, the switch 8 is turned off so that the counting operation of the real time counter is abnormally suspended, as a waveform (E) in FIG. 2, during the periods P.sub.1 and P.sub.2.
Meanwhile, the conventional control apparatus of a real time counter as shown in FIG. 1 is being adapted to the video camera and the VCR (model names are GS-E.sub.2 and R-DDIP each) which are now commercially available from the applicant, Goldstar Co., Ltd. In addition, there is disclosed a prior U.S. Pat. No. 5,193,033 pertaining to the CLOG detection section 30 in FIG. 1.
FIG. 3 is a brief view showing video tracks on the videotape and the position of the head scanning the tracks. In FIG. 3, t.sub.1 stands for the width of the track, and t.sub.2 for the width of the head. In the case that t.sub.1 &gt;t.sub.2, a guard band the width of which is t.sub.1 -t.sub.2 is formed (it represents, in FIG. 3, narrow bands between the tracks which are not hatched). When the head normally scans the track as `a` in FIG. 3, the output of the head (RF signal) is strong, while when the head abnormally scans the track as `b` in FIG. 3, the RF signal corresponding to the track is weak.
Accordingly, in the case of `b` in FIG. 3, when comparing the envelope signal detected at the envelope detector 3 with the reference voltage Vcref, if there appears a signal less than the level of the reference voltage Vcref (a signal appearing under the Vcref at the waveform (A) in FIG. 2), it has an effect on the counting operation. That is to say, even though there are some recorded signals on the videotape, the recorded signals are not read out due to the head's positional deviation. So, as a result, the real time counter suspends the counting operation.
Specifically, in a purely blank videotape, because the level of the envelope signal (waveform (A) in FIG. 2) should be lower than that of the reference voltage Vcref, Vcref must be necessarily established at a lower level. But, in effect, there is a technical limitation that the reference voltage Vcref cannot be set too low.
In practice, because of noise permeation, unless the reference voltage Vcref is set very low, the period that the counting operation is suspended appears as the periods P.sub.1, and P.sub.2 at the waveform (A) in FIG. 2, although a signal really exists. It follows then that counting accuracy is compromised. In other words, in the conventional art, since the counting operation depends only upon the CLOG signal, in the case that the head deviates from the video track as `b` depicted in FIG. 3, the difficulty in accurately detecting the CLOG signal gives the real time counter an operational impediment.